(1) Field of the Invention
The present invention relates to the separation of heavy isotopes of hydrogen, and in particular to a process and apparatus for separating tritium oxide (HTO) from contaminated water by passing the contaminated water through a molecular separation material containing hydration sites, i.e., sites with multiple associated waters of hydration, to selectively replace the waters of hydration with water molecules that include the heavy hydrogen isotopes.
(2) Description of the Prior Art
Nuclear power plants must routinely deal with the replacement and disposal of contaminated water that is laden with heavy isotopes of hydrogen, namely deuterium oxides, tritium oxides and deuterium-tritium oxides. Tritium in particular is radioactive having a half-life of about twelve and one half years emitting beta particles to form helium.
Periodically, the contaminated water from nuclear reactors must be replaced. It has become industry practice to dispose of the old contaminated water by simply discharging it as allowed by their permits and licenses over ground areas. This is stressful to the environment as the deuterium oxides and tritium oxides are now known to have contaminated ground water sources. There appears to be no effective and economically practical means for otherwise disposing of this contaminated water so this practice of disposal continues.
Tritium is a radioisotope of hydrogen which decays to helium3 by beta emission with a half life of 12.3 years. Tritium in the environment exists mainly as tritiated water (HTO) for example as in wastewater discharged from nuclear facilities. A tritium concentration of 1 xcexcCi/L in water, is equivalent to a tritium-to-hydrogen atom ratio of 3.12xc3x9710xe2x88x9213. Deuterium, another isotope of hydrogen with an atomic weight of 2.0, is also present in water at a natural concentration of about 150 ppm or at a deuterium to hydrogen atom ratio of 6.75xc3x9710xe2x88x924.
Tritium is typically found in dilute form as HTO in the oxidized state. The US EPA has set a drinking water standard of 0.02 xcexcCuries/L for HTO. Tritium is used in nuclear weapons systems and is a frequent by-product of nuclear processes. It is estimated that there are 20,000 Curies of tritium at the US DOE""s Hanford site and more at the Savannah River Site (Jeppson 1999, Miller 1999). It has also been estimated that greater than 90% of the total dose from radionuclides to downstream water users is HTO as contaminated aquifers have reached the Columbia and Savannah Rivers. These rivers remain well below the drinking water standard but HTO in these streams is a concern.
A typical nuclear power plant produces 500 Curies per reactor per year and again it has been established that over 90% of the dose entering the environment from these plants is HTO. As with the US DOE sites, lakes and rivers to which these power plants discharge remain below the drinking water standard.
Heretofore, there have not been practical processes for separation of HTO from light water at these low concentrations (parts per billion to parts per trillion or even lower). In high concentrations, distillation and cryogenic processes are practiced. Electrolysis followed by gaseous separation processes is also practical at high concentrations.
Selective adsorption is a common method to attack removal of trace quantities in liquid and gas streams. In this process, the selective site is the hydration region surrounding a cation, typically aluminum +3, anchored to a substrate. Most salts have hydrates and aluminum sulfate is reported to have up to 18 waters of hydration (Lide, 1996). All of these potential hydration sites are probably not accessible in a supported site configuration. It has also been reported that some cations have two spheres of hydration sites with the closer sphere having a higher heat of hydration. In addition to having a high total hydration capacity it is necessary for the system to be selective for heavier water isotopes.
In accordance with the present invention, a process and related apparatus is described for separating tritium oxide (HTO, T2O), i.e., heavy water, from tritiated water. The process and apparatus may also be used in separating deuterium oxide (HDO, D2O) and, deuterium-tritium oxides from waste water. As used herein, water molecules of the formula H2O will be referred to as light water molecules, or simply water molecules, while water molecules in which the hydrogen atoms contain one or two neutrons will be referred to as isotope water molecules, or isotope molecules.
In the described process, a portion of the isotope water molecules are removed from contaminated water, i.e., water containing a small amount of isotope water molecules, through selective adsorption by contacting the contaminated water with a molecular separation material containing sites carrying multiple associated waters of hydration. In the process, isotope water molecules present in the contaminated water selectively replace a portion of the waters of hydration associated with the hydration sites. The molecular separation material can then be separated from the water, reducing the percentage of isotope molecules in the water. After separation, the molecular separation material can be regenerated by removing the isotope molecules, and again used to separate isotope molecules.
Generally, the separation material of the present invention is comprised of either an organic or inorganic high surface area support medium having a plurality of hydration sites, i.e., sites with associated waters of hydration. The effectiveness of the separation material is determined by the number of hydration sites exposed to the contaminated water, and to the number of waters of hydration at each site.
The support structure or medium may be, for example, a polymer, such as polystyrene/divinyl benzene (PSDVB), or polyacrylic/divinyl benzene (PADVB). These polymers are commonly used as supports in ion exchange resins in the preparation of ion exchange resins. The polymer may be functionalized, e.g., sulfonated or phosphonated to provide the sites for attachment of metal or other ions with the required associated waters of hydration. The metal ions may be attached by reaction of the sulfonated or phosphonated resin with a salt, e.g., a sulfate or nitrate salt, in which the metal to be used for the site is the cation. The procedure was applied for RSO3H with 2%, 4% and 12% cross-linking, 60 mesh, 100-200 mesh and 200-400 mesh screened for size.
In a preferred embodiment, Al2(SO4)3 is reacted with the resin to provide the following structure: 
or structures with only one or two SO3 linkages to the resin.
The relative effectiveness of various cations was measured for selective adsorption of tritiated water at low concentration (125 xcexcCi/L) in light water. The substrate was a commercial polyacrylic/divinylbenzene resin loaded with the cations as sulfates at one normal concentration and subsequently rinsed with deionized water until the rinse conductance indicated no change. The cations are ranked in the table below by relative adsorption of HTO per unit wet volume of adsorbate.
It is important to note that the present invention involves the replacement or substitution of the waters of hydration associated with the site, and not the replacement of the cation or anion as is normally practiced in using this type of resin. Thus, while the resins employed are referred to in some instances as ion exchange resins, since this is the purpose for which they are commonly employed, their function in the present invention is the molecular exchange of isotope water molecules with the associated light water molecules. If one begins with a dry resin, the tritiated water will be preferentially adsorbed and held.
Preferably, each hydration site will have at least 5, and preferably from about 7 to about 20 waters of hydration. Various molecules that form associations with water molecules, i.e., waters of hydration can be used in the present invention. The cationic portion of the hydration site may be non-metallic, e.g., an ammonium cation (NH4+), or a metallic cation. Of the metal cations, aluminum is especially suitable due to the large number of waters of hydration associated with aluminum salts. However, other metal cations, such as magnesium, copper, zinc, cobalt or chromium can also be employed. Depending upon the structure of the support and the manner of its production, the anionic portion of the hydration site molecule can be nitrates, sulfates, chlorides, hydroxides, or phosphates.
The molecular separation material may be in various physical forms, so long as a large surface area with metal sites is exposed to the contaminated water. For ease of manufacture and subsequent regeneration, and the availability of a large surface area, the material is preferably in the particulate form, e.g., beads of from about 15 mesh to about 400 mesh. Other physical forms, such as gels, can also be used.
When the substrate is an organic resin, the resin is preferably a highly sulfonated PS/DVB resin with 10 to 12% cross-linking, loaded with aluminum cation. Best results are achieved with resins having the key maximum number of sites per unit volume.
Instead of using a PS/DVB resin as described above as the support for the hydration sites, it has been found in accordance with the present invention that additional advantages are provided by the use of an inorganic support. These supports include aluminas, silicas, and the aluminum-silicates or zeolites. These substrates are treated in much the same manner as the PS/DVB substrates by first functionalizing the substrate, and then attaching metal cations, or by replacing the xe2x80x94OH group with a metal cation with multiple waters of hydration.
That is, acidic or basic alumina or silica, or a mixture thereof, e.g., a zeolite, may be functionalized by reaction of the xe2x80x94H or xe2x80x94OH groups, respectively, with H2SO4, and then reacting the functional group with Al2(SO4)3, resulting in an inorganic polymer having the formula: 
wherein R is alumina or silica, or mixtures thereof, A is an aluminum, magnesium, copper, zinc or cobalt cation, and Y is a sulfate, nitrate, chloride or phosphate cation. Other inorganic supports, such as magnesium silicate could also be used.
Alternatively, the acidic alumina or silica can be reacted with a metal halide capable of attracting multiple waters of hydration to replace the xe2x80x94H group. For example, AlCl3 can be reacted with the acidic substrate to produce an inorganic polymer with the following formula: 
This polymer can be used as the adsorption material, or the polymer can be reacted with Al2(SO4)3 to produce an adsorption polymer with the following structure: 
Inorganic polymers, while performing much like organic polymers in selective attachment of tritium molecules, have additional advantages in rehydration of the polymer. Specifically, after the polymer has been loaded with tritium molecules, the polymer is heated, usually with heated air, to separate the tritium molecules and regenerate the polymer for further use by removing the waters of hydration. Regeneration using organic polymers, such as PS/DVB resins, is a slow process, since waters of hydration are difficult to remove at temperatures that will not degrade the organic polymer, i.e., temperatures below about 140xc2x0 C.
However, with inorganic polymers, the regeneration temperature can be significantly higher, e.g., in the range of from about 140xc2x0 C. to about 300xc2x0 C. This can substantially reduce the regeneration times required, e.g., from about 12 to about 100 hrs. to about xc2xd to about 2 hrs. Therefore, substantially greater efficiencies can be achieved, permitting the utilization of smaller and fewer treatment modules, and a smaller quantity of polymeric material.
In addition, the contaminated water can be heated to a vapor phase prior to adsorption of the tritium molecules, resulting in very rapid adsorption and desorption. Also, at the higher temperatures, the amount of water held nonselectively is dramatically reduced.
The configuration of the apparatus used to practice the process of the invention will vary depending upon several factors, including the volume of water being treated, and the manner of disposal of the water discharge streams. In general, however, the apparatus will include at least one separation chamber, a supply conduit for conveying contaminated water into the separation chamber from a supply source, and a first discharge conduit for removing diluted waste water from the separation chamber. Provision may also be made for periodic replacement of the molecular separation material. The apparatus also includes a means for regeneration of the separation material to remove adsorbed isotope molecules and regular water molecules. For example, the loaded separation material can be exposed to heat to drive off the isotope molecules and the light water molecules by selective evaporation. This desorbed or dehydrated separation material can then be used directly, or rehydrated with light water molecules prior to use.